Are You Smarter Than a Mollusk? Maybe Not

Jessica Leigh Hester of Atlas Obscura did a story about the simplest animal brains. She interviewed Lauren Sumner-Rooney who is an evolutionary biologist at the Oxford University of Museum of Natural History. She has a special interest in chitons.


Typically has 8 plates for a shell called a girdle. These plates can flex. There is some debate as to the true number of species around the world. The claims go anywhere from 500 to 1000 species. The basic exterior surface can be smooth, scaly or spiny. Chitons typically are thought of as “bottom dwellers” living in mostly rocky areas in shallow coastal waters.


A Lined Chiton (Tonicella lineata) on a Puget Sound beach at low tide.


But one can find some in deep waters as well. The anatomy includes a large muscular foot for motion. They are herbivores mostly, but others eat shrimp and small fish. Yet others can eat sponges. The typical algae eating chiton has teeth described as radula. These teeth scrape algae off rocks in order to feed. They can be less than an inch to one foot in length. They have a three chambered heart, copper based blood, complex kidneys and, wait for it, a simple nervous system by which they can fold up into a ball. Presumably, this protects their soft underbelly from predators.


Prof. Sumner-Rooney looked at about a dozen different species in search of some commonalities in the neuroanatomy. She found something. At first glance, the nervous system looks like an 8 runged ladder.


A more detailed cross comparison also found a ring of neurons. This can be thought of as a rudimentary “head” which would coordinate information coming from different parts of the chiton’s body. The types of multi-tasking a mollusk must do lead to the suggestion that more research is needed in order to understand just how complex a deceptively “simple” neural system can be. The questions raised by Prof. Sumner-Rooney should make researchers rethink the relationships between centralized and distributed intelligence.


A related question would be about what, if any, reflexes a chiton has – or other mollusks. Historically, neuroscience drew a tight connection between structure and function. A certain neural structure would correspond to a certain neural function. The idea still lives on, but the growing body of knowledge about neuroanatomy suggests we have to look elsewhere for a correct explanation of the relationship between different neural structures and their associated functions.


Some scientists coined the term “Central Organizing Principle” as an abstract way to deal with the fact that there is no obvious connection between what they are looking at under the microscope and what they see the organism do. Genetically determined, yet adaptive to the environment, chitons pose their own riddles.


Other research on chitons by Alexandre Lobo-de-Cunha, et.al., took a closer look at the digestive glands of two species of chitons. Again, one can be surprised by the complexity in what is supposed to be a simple organism. When algae is the main food source, the body needs digestive enzymes specialized for a range of carbohydrates. A simple, unanswered, question is when does a chiton know it is eating algae? Presumably, there would have to be a feedback mechanism that says “No, dummy! You are eating mud, or scraping a rock.” This alone should make clear that, however rudimentary, there is some form of cognition required in order to survive as a chiton.

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